On a visit last week to our town library, I parked next to a line of ornamental maple trees ablaze with brilliant, sun-struck foliage. Beneath them, a riot of scarlet, crimson and orange leaves covered the ground, free for the taking. (Or so I assumed.) So I grabbed a shopping bag from the car and stuffed it full, a little guiltily—as if I were pilfering rubies instead of dead leaves.